Method for recovering and/or recycling material

ABSTRACT

A method for recovering and/or recycling starting silicon material by crushing the starting material. The recovered or recycled material is melted, and crystals, e.g. as a silicon block, tube, or strip, are grown from the obtained melt. To use starting materials that have a high aspect ratio to be able to convey the same without any problem, broken polycrystalline needle-shaped Si material (material I) containing particles having an aspect ratio A I , 5&lt;A 1 ≦30, is used as a starting material. Material I is crushed so that the crushed particles (material II) have an aspect ratio A II &lt;3. Alternatively, a broken Si wafer is used that is composed of laminar particles which are crushed so that the crushed particles (material III) have an aspect ratio A III &lt;3.

Background of the Invention

This application is a 371 of PCT/EP2007/001943 filed on Mar. 7, 2007.

The invention relates to a method for recovering and/or recycling lumpysilicon starting material by crushing the starting material, whereby therecovered or recycled material is melted, and from this melt, crystalsare grown as silicon block, tube, or strip, for example.

To produce silicon crystals, large waste pieces are normally used, suchas fragments or chunks left over from poly-Si production, sawed-off endpieces of Czochralski or float-zone crystals, edge pieces from discardedblocks, and spherical or splinter-shaped broken material.

With Si-block casting technique, fine Si-granulate is required in orderto fill gaps and hollow spaces during the filling of crucibles, in orderto achieve high filling rates.

With the Czochralski (CZ) production technique, Si-chunks (large pieceswith a diameter of several centimeters) are used for charging. Forpost-charging, some spherical material with a particle size of 0.2-5 mmis also used. Such particles are normally produced using a fluidized bedseparation method. With this material, which is approximately onemillimeter in size, considerably finer gradation is achieved incharging.

Also with the edge-defined film-fed growth (EFG) method, in whichSi-wafers in the form of polygonal tubes are drawn directly out of themelt, a granulate with a particle-size in the millimeter range is used,because due to the crucible volume, finer charging is required. In thestate of the art, spherical material is currently used exclusively.

In addition to this spherical material, there are also irregularlyshaped particles of the corresponding size that can be produced bybreaking them off solid polycrystalline silicon. This material shows apercentage of approximately 0.1% to 10% (mean value 1%) due to theproduction method (pyrolytic growth from the gas phase on a rod), adistinctly elongate structure with a high aspect ratio (length tosmallest upward measurement; hereinafter the simplified ratio of lengthto width will be used). This substance is recognizable from the materialcontaining needles with an aspect ratio (length:width) that is muchhigher than 1, as well as particles with an aspect ratio of 10-30. Thesecan only be filtered out only with difficulty. But the elongatestructure is disadvantageous when conveying through pipes with a smalldiameter or those that have curves and corners.

From WO-A-2005/088680, a method is known for conveying solids particlesof irregular geometry that have a large aspect ratio and can be conveyedthrough pipes by first adding solid particles with a regular geometricalform. In order to ensure unobstructed conveyance, the solids particleswith an irregular geometrical form should have a aspect ratio of ≦3.

Recycled Si-block remainders, chunks, and sections of Cz crystals, dueto their size, create hollow spaces in the bulk material, so that thebulk density is relatively low.

These hollow spaces are therefore filled with finer, and as statedabove, often spherical material.

U.S. Pat. No. 4,084,024 describes a method of producing high puritysilicon. The silicon material produced displays a uniform geometry. Inorder to produce single crystal silicon blocks with the required doping,according to WO-A-86/06764, mixtures of different silicon startingmaterials are used.

According to U.S. Pat. No. 5,304,241, a powder mixture consisting ofsilicon dioxide is used as a filler for a sealing agent for electroniccomponents. In order to obtain the pulverized material, silicon dioxideparticles are crushed using a pulverizer, which is lined with a ceramiclayer in order to avoid contamination. The aspect ratio of thepulverized particles ranges from 1.2 to 1.4.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to develop a method of theinitially cited type, so that starting materials that contain needleswith a high aspect ratio will be suitable, on the one hand, forconveyance without causing problems, particularly through pipes with asmall diameter or pipes with angles, and on the other hand, will besuitable for use in post-charging in melts to the required extentbecause of their bulk density.

In order to achieve this object, the invention essentially provides thatas starting material, either crushed polycrystalline silicon materialthat has at least some percentage of needle-shaped particles (materialI) that have an aspect ratio of A₁ with 5<A₁≦30, is used, whereby thematerial I is crushed in such a way that the crushed particles (materialII) essentially have an aspect ratio A_(II)<3, or a broken Si-wafer isused which consists of laminar particles that are crushed in such a waythat the crushed particles (material III) essentially have an aspectratio of A_(III) with A_(III)<3. Material I can be a silicon extractedfrom the gas phase or a melt.

In accordance with the invention, a silicon material is crushed andbroken in order to obtain a higher percentage of particles with auniform aspect ratio of A_(II) with A_(II)<3. This is done throughtargeted crushing, particularly in a cutting mill. Through theprocessing of the material I, and the smaller and more uniform aspectratio of the material II that is achieved, there is the advantage thattransport through curves and around corners or through a pipe of asmaller diameter is possible without the necessity of adding particleswith regular external geometry. In addition, compared with the materialI, there is a 10% to 20% higher layer density, which enables morecompact filling of crucibles and ingot molds.

Alternatively, there is the possibility of using broken Si-wafer as thestarting material for recycling, using a conveyable Si-granulate ofdoped and/or non-doped laminar silicon material, such as broken,rejected silicon wafer from sawn Czochralski wafers or multi-crystallineblock casting methods, as well as wafers cultured according to the EFGmethod, or corner and edge waste from a polygon resulting from an EFGprocess or another strip-drawing process, for example ESB or the RSGmethod (ribbon growth substrate), or from crystalline wafer generally.The milled broken Si-wafer, whose particles essentially have an aspectratio of A_(III) with A_(III)<3, preferably A_(III)≈1, can be returnedto the wafer-producing loop, whereby recycling based on the inventiveidea is high-purity recycling, as explained below.

In order to provide charging material with the required doping, themixing of recycled starting material with known doping in the requiredscope is provided. For producing material III in particular, it isprovided that broken wafer is presorted and collected separately, forexample in barrels, whereby presorting is done according to thecharacteristics of the starting material.

This is followed by an appropriate classification of the startingmaterials, whereby for example, broken wafer is collected and storedseparately according to resistance group and/or degree of purity. In afurther development of the invention, there is the possibility ofcollecting material based on doping or origin or the original productionmethod, so that the materials whose doping is uniform within a desiredrange, or whose origin or production method is the same, are assigned toone class. Aside from that, the allocation and class type are defined bythe intended purpose.

In this way, a corresponding material class can be defined, even whenthe material contains macroscopically visible contaminants. Surplussilicon resulting from the cutting of an EFG octagon, for example, cansometimes contain elements that cannot be removed immediately, at thestart of a process. However, these elements can be removed beforefurther processing, so that the material is purified before the nextprocess.

The systematic collection of starting material by classes can also bedone after crushing, in order to subsequently be able to produce acharging material with the desired qualities through the mixing ofmaterials from different classes.

It is provided in particular that the material III, which features anessentially polygonal laminar geometry, has a maximum length L of 0.1mm≦L≦10 mm, preferably 0.5 mm≦L≦3 mm and/or a thickness D of 0.5 mm≦D≦3mm, preferably 0.1 mm≦D≦0.6 mm.

As material III, a material should preferably be used that has a volumeresistance R_(v) with 1 μΩcm≦R_(v)≦2000_Ωcm.

A further development of the invention provides for the crushing to bedone by crushing machines that preferably operate with a rotatingcrushing tool. For flat material samples, such as broken wafer, acutting mill is preferably used, regardless of the wafer size.

A cutting mill is also suitable for relatively flat and needle-shapedgranulate with dimensions ranging from 1 cm to 3 cm, i.e. for materialI, or material II. Large or voluminous, solid waste pieces willpreferably be conveyed to jaw crushers, in order for them to crush thematerial to a size that is then suitable for a cutting mill.

Regardless of the means employed, crushing takes place to an extent thatwill allow subsequent reuse. The crushing process is carried out in sucha way that the production of large amounts of silicon dust or anaccumulation of silicon dust in the product is avoided. Such silicondust would result particularly from grinding processes, so grinding andabrasion is avoided in the crushing process. In the case of chopping andcrushing of materials, however, there is little dust accumulation.

Irrespective of the means used, the crushing machine should reduce theparticle size of the starting material in such a way that the particle,in the direction of its longest extent, measures no more than 10 mm, andpreferably 5 mm.

In order to ensure that there is no uncontrolled contamination of therecycled material during crushing—this can occur through erosion of theinternal walls of the crushing tool or cutting mill—it is provided thatat least the internal walls of the crushing device are lined with asilicon material or a mechanically stable and scratch-resistant ceramiccompound, such as SiC, SiN, or WC (solid). Layers can also be applied bymeans of a thermic spray-coating process, plasma spraying, a vacuumprocess, etc. Preferably, however, these are solid walls with athickness of between 0.5 mm and 50 mm, preferably between 5 mm and 10mm, made of pure silicon or sintered or solid CVD-coated SiC or Si₃N₄.The linings can be applied to the internal walls with a suitable fixingmaterial such as an adhesive, which ensures that if the linings wearoff, only material from the wall lining will be mixed into the materialto be crushed and recycled, and that there will be no metalcontamination from the body of the crushing machine itself.

An alternative proposal provides for the wall to be lined with anabrasion-resistant synthetic material, such as Vulkollan®, Teflon®,Halar® and/or polyoximethylene.

In order to avoid contamination of the recycled material, it is furtherprovided that the crushed starting material be discharged from thecutting mill without passing through a screen, such as a perforatedmetal plate, as is otherwise usual.

In that way, contamination is avoided which, in the state of the art,occurs because the size of the crushed material is determined by theapertures in a perforated metal plate or screen through which thematerial is discharged from a cutting mill. The corresponding plate orscreen is subject to wear, because the crushed Si-particles constantlyabrade material from the perforated metal plate or screen, and from thewear protection coating applied to it.

The percentage of particles of maximum extension (length in material IIor a diagonal in material III) depends on the rotational speed of thecutting or rotor knife (FIG. 6). In order to achieve this, thecorresponding crushing machines are operated at higher rotational speedsthan those of normal operation, whereby preferred rotational speedsbetween 500 revolutions/min. and 4000 revolutions/min. are specified inthe case of a cutting mill, and for producing material II, preferably3000 to 3500 RPM, and for producing material III, preferably 2000 to2500 RPM.

According to an additional proposal of the invention, it is providedthat material is crushed by putting the starting material in areceptacle such as a drum made of a synthetic material, and thereceptacle is then put in rotary motion. For this purpose, thereceptacle, such as a barrel, can be placed on a roller block and thenmade to revolve about its axis using the roller block. Due to therevolving motion, the material in the receptacle will be put underconstantly varying load and thus crushed. The crushing effect can beintensified by inserting a round Czochralski-drawn solid cylinder, forexample, or an angularly-sawn solid body of silicon into the receptacle,whereby the solid cylinder, or the solid body, because of its weight androlling motion, will put constant load on the starting material andcrush it.

Owing to the use of synthetic material for the receptacle, the crushedmaterial has a high degree of purity, because no metallic materialenters it from the walls. The fine dust that results from grindingmotions should be immediately removed from the interior of thereceptacle during the crushing process. This can be done by covering oneor more of the openings in the receptacle with a screening fabric forthe largest allowable particle (for example, mesh widths of 1 to 3 mm).In that way, the fine particles fall through the mesh and out of thereceptacle and cannot be crushed to an even smaller size. The productionof fine dust is therefore prevented.

In addition, it is possible to continuously add material to thereceptacle, so that continuous operation is possible. It is provided inparticular that an automatic feeding device be used, with continuousfeeding from the axis of symmetry of the revolving receptacle, wherebySi, for example, can be fed in the amount required by the specifiedrotational speed.

Immediately after crushing, the material is divided into suitablegrain-size fractions by a sieving machine, for example 0≦KG 1(under-dimensioned particles)≦0.3 mm/0.3 mm≦KG 2 (product fraction)≦3.0mm/3.0 mm≦KG 3 (over-dimensioned particles).

Other details, advantages, and characteristics of the invention becomeevident not only from the claims and the characteristics derived fromthem, individually or in combination, but also from the explanationssupplied with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 Aspect ratio of material to be recycled, or recycled material,

FIG. 2 Photographs of grain shape and etching figure of a material to berecycled,

FIG. 3 Photographs of grain shape and etching figure of a recycledmaterial,

FIG. 4 Photographs of grain shape and etching figure of an additionalmaterial,

FIG. 5 Photographs of grain shape and etching figure of an additionalmaterial,

FIG. 6 A schematic representation of grain-shape distribution dependingon the rotational speed of a cutting tool, and

FIG. 7 A schematic diagram of a cutting mill.

DETAILED DESCRIPTION OF THE INVENTION

The characteristics of the materials to be recycled, or the materialsthat have been recycled in accordance with the invention, said materialsbeing intended for charging melts from which silicon crystals will begrown, will be explained with reference to figures or to the photographsand etching figures they contain. The crystals can be produced accordingto the Czochralski method or the EFG process, to name just a fewproduction methods as examples.

According to the inventive teaching, irregularly shaped particles thatcan be produced by breaking them out of solid polycrystalline silicon,undergo further processing. Due to the production method (pyrolyticgrowth from the gas phase on a rod), the material has a distinctlyelongate structure with a high aspect ratio (length to smallestmeasurement, hereinafter defined for the purpose of simplification as,length to width). The corresponding materials, which are produced fromsolid polycrystalline silicon, are characterized by the fact that thematerial contains needles that have an aspect ratio (length:width) ofthe needles that is much higher than 1, and that there are thus fewparticles with an aspect ratio of 10-30. These can only be screened outwith difficulty, and this means that during the conveyance of thematerial that is called material I in this context, the correspondingparticles have an obstructive effect, particularly when material isconveyed through pipes with a narrow diameter or through pipe bends andaround corners.

In accordance with the invention, the material I is crushed and furtherprocessed in order to obtain a higher percentage of particles with auniform aspect ratio A_(II)<3. This is done through targeted crushingwith a cutting instrument such as a cutting mill.

In order to illustrate the differences in the aspect ratios of materialsI and II, reference is made to FIG. 1. This shows the relative frequencyof the particles according to aspect ratio. Verification itself is doneusing an optical particle-analysis device, even though the differentgeometric dimensions are already visible optically. One can see that thestarting material composed of crushed polycrystalline silicon contains anot inconsiderable percentage of particles whose aspect ratio is higherthan 10, lying particularly in the range of 10 to 30. If the material iscrushed in the manner described below, the aspect ratio can be reduced,namely to the value A_(II)<10. This results in better conveyancecharacteristics for the material II compared with material I.

Crushing the material I also produces a higher bulk density for materialII and thus the possibility of more compact filling of a crucible oringot mold. An additional characteristic of the material II is seen inthe etching figure of an abraded silicon sample. FIG. 2 shows an abradedand chemically etched Si-particle, which is characterized by finelybranched grain boundaries in the material. The material I is obtainedthrough gas phase precipitation, whereby the grain-boundary gaps aredependent on precipitation conditions. The material II, which is derivedfrom material I by crushing material I, can be recognized by the typicalmicrostructure, which is identical to that of material I, as can be seenfrom FIG. 3. As a result, the differences between material I andmaterial II are not in the microstructure, but rather in grain shapealone, whereby the material II has the smaller aspect ratio.

The crushed particles of material II, which are made from material I,predominantly display a less needle-shaped form and a rounderappearance, as FIG. 3 illustrates.

In accordance with the invention, however, not only polycrystallinesilicon produced through pyrolytic growth from the gas phase is used foraddition to a melt, but also material produced from broken Si-wafer.This involves a non-conveyable Si-granulate made from doped and/ornondoped laminar silicon material, such as broken or unbroken discardedsilicon wafer that measures, for example, 156 mm×156 mm, 125 mm×125 mm,or 100 mm×100 mm, or fragments of the same, produced from sawnCzochralski wafers, or multi-crystalline block-cast wafers, as well asfrom wafers grown using the EFG process, and from corner and edgecuttings from a polygon from the EFG process, from other strip-drawingprocesses (for example ESP) or the RGS (ribbon growth substrate) method,as well as generally from single and multi-crystal wafers. According tothe teaching of the invention, this wafer waste can be effectivelyreintroduced into the production cycle.

The aspect ratio can be determined with an optical particle analysisdevice. This shows an even lower aspect ratio (length:width) comparedwith the material II, as seen in FIG. 1. Values for the aspect ratioA_(III) are normally A_(III)<3, which is only exceeded in rare cases.

The material III is also characterized by a virtually constant thicknessof at least a large percentage of the particles. In addition, throughmechanical polishing and chemical etching, a typical grain and defectstructure can be seen in material III, which is very obviously differentfrom those of materials I and II, as a comparison of FIGS. 2 and 3 withFIGS. 4 and 5 demonstrates.

FIG. 5, left, shows material III, which is produced from recycled EFGwafers, as an example of a material drawn from a melt for furtherprocessing. The material has a slice thickness ranging from about 50 μmto 450 μm, preferably 250 μm-350 μm. It breaks into rectangular partsand displays the twin-grain formation that is very typical of the EFGprocess in a large number of particles (FIG. 4, right).

FIG. 5 in the left image shows a material III for further processing,which is produced from multi-crystalline wafers and those produced, forexample, according to the block-casting method. The material has a waferthickness of between 50 μm and 450 μm, preferably 250 μm-350 μm. Itbreaks into polygonal rectangular and triangular pieces and displays theformation of grain boundaries that is typical for the ingot-castingmethod, as the etching figure on the left demonstrates. It should benoted in this context that this grain-boundary structure is less oftendetected in the material, as the size of the crushed particles is belowthe grain-size typical of the material, of about 1 cm to 2 cm. Becausethe material preferably breaks at the grain boundaries, the grainstructure can consequently be determined in only a few pieces in areaswith a very small structure in the millimeter range.

Due to the polygonal form of the particles of the material III, a highbulk density can be achieved, that is 15% to 20% above the value ofmaterial I.

The table below shows the bulk and tap density of the materials I, II,and II, which are to be understood purely as examples.

TABLE 1 Bulk density and tap density Compact density Loose bulk (bulk(tap density) Material Aspect ratio density) [g/cm³] [g/cm³] Material I,screened 0.94 +/− 1% 1.15 +/− 4% (0.5-2.0 mm mesh width) Material II,1.15 +/− 1% 1.19 +/− 4% screened (0.5-2.00 mm mesh width) Material III,  1.15 +/− 1.5%   1.35 +/− 0.2% screened (0.5-2.00 mm mesh width)

From the table, it can be seen that the bulk and tap density of recycledsilicon of the material III results in flat wafers with a comparablescreen fraction and 15 to 20 percent higher density than material I.

The materials I and II, due to the production technique (high-purity gasphase precipitation), are usually nondoped, although precipitation ofdoped polycrystals is in principle possible in the state of the art.

If remelt material is used as the starting substance for material I, thedoping must be considered and determined beforehand. In this case, thematerials I and II can be p- or n-conducting, depending on the doping ofthe starting material.

The material III can contain nondoped or doped, p-conducting and/orn-conducting particles. If the type and degree of the dopingconcentration of the starting material is known, the desired dopingconcentration can be adjusted through suitable mixing of grains ofdifferent concentration. The doping concentration can be determined bymeans of automated test assemblies (such as 4-peak measurement, thermopower, microwave reflection, IR absorption), but also through chemicalanalysis such as mass spectroscopy, AAS (atom absorption spectroscopy),TXRF (total reflection x-ray fluorescence analysis).

The desired doping of a material is achieved by adding calculatedamounts of differently doped or nondoped material. In this way, it ispossible to make a raw material with a desired overall dopantconcentration by mixing different components. In this way, remainders ofhighly doped wafers, for example n, p>5×10¹⁸ cm⁻³, can be used directlyto dope nondoped melts.

In order to produce material III, broken wafer of every possible type iscollected in drums. It has been shown to be advantageous if materialsare collected with the desired characteristics in mind in the earlystage of accumulation, because at that point, knowledge of materialqualities is still present.

Accordingly, suitable classification of the material should beundertaken during collection, so that material can be collected in anorderly way, classified according to resistance groups for example, oraccording to doping percentage, or purity class. In that way, forexample, materials can be collected by resistance groups, wherebyresistance conforms within a factor of 2. The division into classes andclass characteristics will be determined by the intended end use of thematerial.

Even when the material contains macroscopically identifiable impurities,a corresponding material class can be determined for it. For example,silicon left over from cutting EFG octagons sometimes containscomponents that cannot be immediately removed at the beginning of theprocess. However, these components can be removed before furtherprocessing, and thus the material can be purified before the processcontinues.

Through targeted collection of starting material according to classeswith the same characteristics, material with the desired characteristicscan be produced after crushing and mixing of this material.

Crushing takes place in a crushing machine, which preferably operateswith a rotating crushing tool. In the case of flat material samples,such as broken wafer, a cutting mill will be used, irrespective of wafersize.

The cutting mill is suitable for use with relatively flat andneedle-shaped granulate measuring up to 1-3 cm. For large or voluminous,solid waste pieces, jaw crushers, for example, are more suitable, bymeans of which the material can be crushed to a size that can beprocessed by the cutting mill. The object of this crushing process is toreduce the material only to the size at which it can subsequently beused. During the crushing process, the material is therefore reduced todimensions ranging between a few millimeters and a few centimeters. Theobject of the crushing process is a grain size ranging from 0.1-20 mm,and preferably 0.3-3.0 mm.

The production of silicon dust should be avoided as far as possible, or,action should be taken to counteract the accumulation of Si-dust in theproduct. Si-silicon dust is often produced by grinding processes, whichis why abrasion and grinding should be avoided during the productionprocess. On the other hand, chopping and breaking of materials ispreferred, as there is then little dust production.

Because silicon, quartz glass, and ceramics are brittle and hardmaterials, there is a risk of contaminants entering through wear on theside walls of the crushing instrument, such as a cutting mill. As astandard measure, all wear-protection layers, for example tungstencarbide, are applied to the mill walls by means of thermal sprayingmethods or plasma spraying methods. However, these layers are worn offby hard materials like silicon during the crushing process, which leadsto contamination concentrations of 1000 to 10,000 ppba of Fe, Ni, andWC, for example, depending on the structure of the wear-protectioncoating. The surface of the perforated screen that is normally mountedon the machine in order to screen out the crushed material isparticularly subject to wear.

The solution to this problem is provided by coating the internal wallsof the mill with a silicon material or a mechanically stable andscratch-resistant ceramic compound such as SiC, SiN, WC, or a hard metal(solid). Layers can naturally be applied using thermic sprayingprocesses, plasma spraying in a vacuum, etc.

Better suited, however, are solid walls with a thickness of 0.5 to 50mm, preferably 5-10 mm, made from pure silicon or sintered or solidCVD-precipitated SiC or Si₃N₄. When the material to be crushed consistsof quartz glass, the wall lining consists of quartz plates. These walllinings are affixed to the wall with a strong adhesive and ensure thatwhen there is abrasion of the wall, only the material making up the walllining, but no metal contaminants, is introduced into the crushedmaterial.

However, there is also the possibility of coating the wall with anabrasion-resistant synthetic material, such as Vulkollan®, Teflon®,Halar® and/or polyoximethylene.

Normally, in the case of a cutting mill, the size of the material to becut is determined by the apertures in a perforated plate. However, sucha plate is subject to wear, as the Si-particles are scraped across it,resulting in continual abrasion of the material making up the perforatedplate and the wear-resistant coating applied to it. It is a feature ofthe invention that operation without the perforated plate, as it issubject to particularly heavy abrasion and can only be provided with ahard-material coating, not with a solid wall lining.

In one embodiment of the invention, therefore, the perforated plate isdispensed with. The size of the particles in the material that is cut iscontrolled instead by the rotational speed of the rotor knife. Thisresults in a cutting method of higher than normal rotational speed. Thedependence of the product of the high-speed rotational cutting mill ishowever strongly influenced by the shape of the grains put into it,particularly the grain thickness, but also the length of the grain.

The relatively large and thin charging material, broken wafer, which isused for producing material III, achieves higher yields even at lowerrotational speeds than the smaller and thicker granulate (material I),from which material of the type II is produced (FIG. 6). The yield isadjusted primarily by means of the cutting-knife interval.

In general, the product-fraction yield increases with rotational speed,and the percentage of over-dimensioned particles decreases. The exactdependence for different materials must be determined for individualcases respectively.

Nevertheless, FIG. 6 basically reflects the conditions that apply to thechange in product fraction and over-dimensioned particles of individualmaterials, which are dependent on the rotational speed of the cuttingtool. The curves 1 and 2 apply to material III. The rising curve 1 showsthe change relative to the product fraction, which increases as therotational speed of the cutting tool increases. Accordingly, the curvemarked 2 is characteristic of the undesirable over-dimensioned particle,i.e. the percentage of over-dimensioned particles decreases with anincrease in the rotational speed.

Regarding the materials I/II, the curves 3 and 4 illustrate that theproduct fraction also increases with a higher rotational speed (curve3), whereas the percentage of over-dimensioned particles (curve 4)decreases.

The more easily the material cleaves, the lower the rotational speedsthat are necessary in order to obtain a high percentage of the desiredparticle size. The smaller the material that is introduced, in terms ofmaximum length, the higher the rotational speed must be in order toachieve the desired product fraction, and in that way minimizeover-dimensioned particles.

The geometry of the material also determines the particle-sizedistribution achieved through the crushing process. Materials with ahigh aspect ratio require higher rotational speeds. Likewise, smaller,less easily cleavable particles require higher rotational speeds.

With an operating method that does not involve a perforated screen, thefine-particle fraction (under-dimensioned particles) can be drasticallyreduced to values under 1%. This is highly advantageous, because thefinely crushed material, with grain sizes under 0.5 mm, due to its largesurface area, absorbs more contaminant particles that adhere to thesurface. With increasing rotational speed, and without a perforatedplate, it was possible to increase the product-fraction yield toapproximately 80%.

The purity of the product is significantly better without a perforatedplate. Finally, less contaminating material is introduced into the waferfor two reasons:

-   -   1. Abrasion due to the perforated plate disappears entirely, so        that a large portion of the contaminants are not introduced.    -   2. Due to the smaller percentage of under-dimensioned particles,        there is less surface area.

Table 2 shows the improvement effect achieved by making certain changes.Lining the walls with silicon produces a significant reduction in thecontamination concentrations of Fe and Ni, i.e. the materials that makeup the cross beaters of the cutting mills. WC is also embedded in Fe andNi. Dispensing with the perforated plate coated with WC results in areduction in the tungsten-carbide concentration that is under the limitof detection. The WC is thereby embedded in a Fe, Ni-layer.

TABLE 2 Purity of products With wear- With Si-lining protection (flame-on the walls With Si-lining and sprayed tungsten and perforated withoutperforated Contamination carbine coating) plate plate Fe [ppba] 1000 200<30 Ni [ppba] 200 50 <10 W [ppba] >1000 1000 <5

The material purity achieved in this way is high enough to producesolar-cell efficiency of more than 14%, to 15%.

Another way to crush material is to put the silicon into a plastic drumon a roller block, the drum then revolving slowly due to the action ofthe roller block. Because the drum revolves, the material is subjectedto constantly changing load, which breaks it up. The breaking effect canbe markedly intensified by inserting a round, CZ-drawn, solid cylinderinto the drum, or a jaggedly sawn, solid silicon body, because thecrystal is constantly subjected to loads and breaks due to its weightand the rolling movements of the Si-material.

This material also has a high degree of purity, because no metallicmaterial from the walls enters it. During the crushing process, finedust resulting from grinding movements is removed as quickly as possiblefrom the interior of the drum.

This is achieved by openings in the body of the drum that are covered byscreening fabric with the mesh width of the largest allowable grain (forexample, a mesh width of 1 mm-2 mm, preferably 3 mm). The fine grainsfall through the mesh and out of the drum and cannot be ground anyfiner, and are available for the rest of the process. An automaticfeeding device can be used, feeding from the axis of symmetry of therevolving receptacle. In this case, as much Si is added to the drum asneeded in order to maintain continuous operation at a specified speed.

Immediately after crushing, the material is divided into suitablegrain-size fractions by a screening machine, for example 0≦KG 1(under-dimensioned particles)≦0.3 mm/0.3 mm≦KG 2 (product fraction)≦3.0mm/3.0 mm≦KG 3 (over-dimensioned particles).

The concepts of material recycling, or recycled material, mentionedabove also include the recovery of materials that are melted in order togrow crystals. In other words, the starting material does notnecessarily have to be previously used silicon material.

In addition, the invention is not restricted to the material that iscrushed into materials II and III being added to a melt, even thoughthat is preferably the case. Rather, the invention also applies tomaterials II and III, which are put into a crucible, without a meltalready being in it.

FIG. 7 is a schematic diagram of a cutting mill 10, by means of whichthe materials I and III are crushed to an extent such that the desiredaspect ratio of less than 3 is achieved, whereby the production of dustthat causes contamination is minimized. At the same time, there is thepossibility of any dust produced being vacuumed up into explosion-proofsuction devices, for example.

The cutting mill 10, which in the schematic depiction is shown with acuboid interior space 11 that merges into a funnel 13, features ahousing 12 that consists of metal, preferably aluminum. An interiorspace 11 with a cuboid geometrical shape is formed by four approximately2 cm thick wall plates 14, 16, which are screwed to the housing 12. Theinternal walls of the housing 12, i.e. the plates 14 and 16, are linedwith about 5 to 10 mm thick silicon plates 18, 20, 22, 24. This can bedone by gluing the silicon plates 18, 20, 22, 24 onto the aluminumplates 14, 16. This requires that the interior surfaces of the walls 14,16 are even.

In the interior space 11 that is surrounded by the plates 14, 16, acutting tool 28 is rotatably arranged. The cutting tool 28 is arotational body 30 that takes the form of a multi-sided column thatpreferably has a triangular, quadrangular, or hexangular base area. Thelongitudinal surfaces of the rotational body 30 are clad with 5 mm to100 mm thick silicon plates 32, 34, 36. In the embodiment shown, therotational body takes the geometrical form of a triangular column.

Cutter bars 38, 40, 42 are affixed to the silicon plates 32, 34, 36,said cutter bars preferably consisting of solid tungsten carbide orsilicon-infiltrated hard material. The cutter bars 38, 40, 42, arepreferably screwed to the rotational body 30 by means of screws. Thesilicon plates 32, 34, 36 contain the corresponding boreholes. The screwheads are countersunk into the blades 38, 40, 42, and the exposedsurfaces are covered with Teflon tape.

Further, the blades 44 and 46 extend from the lateral walls, which runparallel to the longitudinal edges of the rotational body 30 and aremarked with the reference numerals 14 and 16, or from their liners 18,20, 22, 24. The blades likewise consist of solid tungsten carbide andare directly secured to the lateral walls 16 and 18.

The gap between the blades 44, 46 and the longitudinal edges of therotational body 30 that moves past them, or the cutter bars 38, 40, 42that emerge from said edges, can be adjusted. For this purpose, theblades 44, 46, which feature a cuboid geometrical form, are welded tosteel parts 50, 52, which are affixed by means of screws that pass fromthe outside through the housing walls 14, 16. The gap can then beadjusted by means of spacers that are not further described.

The blades 44, 46 are positioned or countersunk between the linings 18,20, 22, 24, so that the material that is to be crushed or broken, comesinto contact only with silicon and tungsten carbide.

In addition, the cutter bars 38, 40, 42 and/or the blades 44, 46 thatemerge from the rotational body 30 should be beveled to about 400 μm, inorder to avoid nicking.

In contrast to conventional cutting mills, at the outlet of the funnel13 there is no perforated plate, which could otherwise be a source ofcontamination.

The rotational body 30 itself rotates at high speeds ranging between 500rpm to 4000 rpm, in order to achieve the desired breaking or crushing ofthe materials that are to be added.

The advantage of the inventive arrangement is an obviously smallerportion of under-dimensioned particles (smaller than 0.3 to 0.5 mm),which are usually highly contaminated, and must be discarded. Without aperforated plate, the percentage is between 1% and 10%, whereas when theusual perforated plate used, the portion of under-dimensioned particleswould lie between 10% and 20%, as measurements have shown.

In order to crush the materials I to III, they are added via a headsidefunnel 54. In the area of the funnel 54, the walls are also lined withsilicon plates that are between 5 mm and 10 mm thick.

While the housing 12 is preferably completely lined with siliconmaterial, it is also possible that less-stressed surfaces can haveplastic plates. The corresponding plastic plates are also glued orscrewed into place.

The material that falls through the cutting mill 10 is chopped betweenthe blades 38, 40, 42, and 46, 48, which are composed of tungstencarbide, and then falls into the lower funnel 13, which is also linedwith silicon, silicon carbide, or with silicon-infiltratedhard-material, or with plastic plates.

From there, the material can be conveyed to a screening machine, whichscreens out the fractions (for example, under-dimensioned particles <0.3mm; product fraction 05 mm to 3.0 mm; over-dimensioned particles >3 mm)that are required for the subsequent processes. The corresponding valuesare just examples and can vary depending on the case of use. The dustthat collects is suctioned away.

Depending on the dimensions of the cutting mill 10, throughputs of 30kg/h, 100 kg/h or more are possible.

1. A method for crushing starting material containing silicon so thatthe crushed material can be melted to allow silicon to be crystallizedfrom the melt, comprising the steps of: crushing starting material,wherein the starting material is either: (1) broken polycrystalline atleast partially needle-shaped Si-material (material I) comprisingparticles having an aspect ratio of A₁, where 5<A_(I)≦30, whereinmaterial I is crushed so that the crushed particles (material II) havean aspect ratio of A_(II)<3; or (2) a broken Si wafer comprising laminarparticles which is crushed so that the crushed particles (material III)have an aspect ratio of A_(III) with A_(III) <3; melting the crushedstarting material to form a melt; and crystallizing silicon from themelt.
 2. A method according to claim 1, wherein the material I iscrushed so that the material II has a bulk density that is 10% to 20%higher than that of material I.
 3. A method according to claim 1,wherein the material I is crushed in a cutting mill.
 4. A methodaccording to claim 1, wherein the material to be fed into the melt has adesired doping as a result of mixing specified amounts of recycledstarting materials with known doping.
 5. A method according to claim 1,wherein the starting material used to produce the material III iscollected separately according to material characteristics.
 6. A methodaccording to claim 1, wherein the crushing of the starting materials isdone through the use of a rotating crushing tool that that breaks up,chops, or cracks the starting material.
 7. A method according to claim6, wherein a cutting mill is used as the crushing tool.
 8. A methodaccording to claim 6, wherein walls of the crushing tool that come intocontact with the starting material are configured in such a way that alining or coating is provided that prevents or essentially preventscontamination of the material to be crushed.
 9. A method according toclaim 6, wherein at least the interior walls of the crushing tool have alayer made of Si, SiC, SiN and/or solid WC.
 10. A method according toclaim 9, wherein, as a coating, solid walls are used with a thickness of0.5 mm to 50 mm that are made of pure silicon or sintered or solidCVD-precipitated SiC or Si₃N₄, or WC.
 11. A method according to claim 6,wherein the crushed starting material is removed from the cutting toolwithout the use of a screen.
 12. A method according to claim 6, whereinthe percentage of useable material II or useable material III isdetermined by the rotational speed of the rotating crushing tool.
 13. Amethod according to claim 6, wherein in order to increase the percentageof useable material II or material III, the rotational speed RPM of thecrushing tool is set at 500 RPM to 4000 RPM, for extracting the materialII, and at 2000 RPM to 2500 RPM for extracting the material III.
 14. Amethod according to claim 1, wherein the material III, essentiallyhaving a polygonal, laminar geometry, has a maximum length L of 0.1mm<L≦10 mm, and/or a thickness D of 0.05 mm<D≦3 mm.
 15. A methodaccording to claim 1, wherein, as material III, a material is used thathas a volume resistance R_(v) with 1 Ωcm<R_(v)≦2000 Ωcm.
 16. A methodaccording to claim 6, wherein the walls of the crushing tool are linedwith an abrasion-resistant plastic.
 17. A method according to claim 1,wherein the starting material is placed in a cylindrical receptacle,such as a drum, which is revolved in order to place alternating stresson the starting material.
 18. A method according to claim 1, wherein, inaddition to the starting material, a solid body made of Si that acts onthe starting material is put into the receptacle in addition to thestarting material.
 19. A method according to claim 1, wherein a solidcylinder or a four-cornered column is used as the solid body.
 20. Amethod according to claim 1, wherein a receptacle with an openingcovered by a screening fabric is used, through which opening the crushedstarting material is removed.